The use of intumescent material in cable ducts, for the purpose of preventing the duct from, in effect, becoming a flue for the transmission of smoke, heat and flame from one area to another, is known. Thus, for example, in non-ventilated cable raceways, such as in the so-called under-floor duct commonly used to house cables, telephone lines, electrical wires, etc., in office and apartment buildings, where it is desired to help prevent transmission of damaging effects of fire within the raceway from one section of the building to another, intumescent materials are known to have been utilized.
By an intumescent material is meant one which enlarges, swells or bubbles upon exposure to heat above pre-determined levels. Thus, for example, so-called "FLAMAREST 1600," as marketed by Avco Systems Division of Lowell, Mass., is an intumescent, epoxy coating, containing an intumescent component designed for both interior and exterior use and many industrial applications. It is a two-component, catalyzed, epoxy resin which fuses into a porcelain-like shield to protect the substrate while providing a highly efficient barrier against flame and heat.
Typically, such coatings may be applied in a thickness of 20 to 25 mils. As the coating is exposed to heat at a preselected level, for example, 500.degree. F., the resin softens and the intumescent material begins to change state and evolves from a high density film to a low density "intumescent char," wherein a multiplicity of air cells in the char act as insulators and keep the substrate cool. As an intumescent mechanism, the foregoing phenomenon may be completed in a matter of seconds, for example, 30 seconds or so, after the coating is exposed to heat and/or fire. By positioning such material in a confined passageway, such as the interior of a cable raceway, it is possible to have the intumescent char traverse the cross-section of the passageway such that, upon completion of the heat exposure cycle of the intumescent material, the passageway becomes effectively blocked. This concept has been adopted in the design of enclosed under-floor ducts, for example, as hereinbefore described.
In addressing the question of utilizing intumescent material in generally open, ventilated devices such as cable trays, it must be kept in mind that the problems involved are significantly different than those in generally non-ventilated structures, such as cable raceways and the like.
Typically, one form of cable tray is made as a flat, hung-support member, having a multiplicity of holes for the liberal convection of air therethrough, so that heat generated in the cables in the normal course of use may be easily and adequately dissipated. However, this very feature of enhancement of convection in normal circumstances becomes undesirable in extreme cases such as fire in the ambient region in which the trays are located because, inherently, it tends to enhance the passage of heat and flame into, through and around the areas in which the cables are positioned. In the past, where the consequences of destruction of cables due to these phenomena were mainly mere power losses, concern was somewhat less than now, when the destruction of cables can have the effect of rendering inoperative control, servo, and other mechanisms and devices, which may be critical to health or safety, as, for example, reaction control devices, in nuclear-reactor installations. It might be thought that to address this problem by coating the inside of the holes in such cable trays with intumescent material, would suffice, with the idea that upon exposure to heat, the material would expand and shut off the convective paths. However, the heat from a fire in an ambient region can be so intense, and the consequent "flue effect" through the ventilating holes in a direction normal to the tray can be so pronounced, that by the time a sufficient amount of time has passed for the intumescent material to begin to react to the sudden heat rise, the rate of convection through the holes, coupled with the relatively fragile "char" state of the intumescent material as it approaches its fully expanded condition, can cause the blocking to be inhibited, or even totally precluded.
Accordingly, it is an object of this invention to provide a means for selectively and automatically restricting the flow of heated air through structures, which are intended to encourage free ventilation under usual circumstances.
Another object of this invention is to provide such means in a fashion most likely to achieve effective restriction of the flow of heated air under extraordinary conditions.
Yet another object of this invention is to provide means to satisfy the foregoing objectives, which is structurally sound from an engineering standpoint for its intended functional use, such as supporting cables, and, at the same time, is structurally simple and comparatively inexpensive to produce.
Still another object of this invention is to provide means to satisfy the foregoing objectives using materials and structures which are reliable and effective.